Photodynamic therapy for pythiosis

Background –  Pythiosis is a life‐threatening disease caused by Pythium insidiosum . Photodynamic therapy (PDT) is an alternative treatment to surgery that uses the interaction of a photosensitizer, light and molecular oxygen to cause cell death. Objectives –  To evaluate the effect of PDT on the in vitro growth of P. insidiosum and in an in vivo model of pythiosis. Methods –  For in vitro studies, two photosensitizers were evaluated: a haematoporphyrin derivative (Photogem ® ) and a chlorine (Photodithazine ® ). Amphotericin B was also evaluated, and the control group was treated with sterile saline solution. All experiments (PDT, porphyrin, chlorine and light alone, amphotericin B and saline solution) were performed as five replicates. For in vivo studies, six rabbits were inoculated with 20,000 zoospores of P. insidiosum , and an area of 1 cm 3 was treated using the same sensitizers. The PDT irradiation was performed using a laser emitting at 660 nm and a fluence of 200 J/cm 2 . Rabbits were clinically evaluated daily and histopathological analysis was performed 72 h after PDT. Results –  For in vitro assays, inhibition rates for PDT ranged from 60 to 100% and showed better results in comparison to amphotericin B. For the in vivo assays, after PDT, histological analysis of lesions showed a lack of infection up to 1 cm in depth. Conclusions and clinical importance –  In vitro and in vivo studies showed that PDT was effective in the inactivation of P. insidiosum and may represent a new approach to treating pythiosis.